An LED driver circuit and a method prevent LED turn-off flash when input power is lost to the driver circuit. The driver circuit includes a DC-DC converter that provides an LED drive voltage to an LED load. A voltage drop sensing circuit detects the loss of input power and discharges a filter capacitor that provides operating power to a controller in a DC-DC converter. The controller turns off to halt the operation of the DC-DC converter before the voltage provided to the LED load decreases to a turn-off threshold of the LED load. The DC-DC converter cannot recharge a load capacitor across the LED load. Thus, once the LEDs in the LED load turn off, the LEDs remain off until the input power is restored.
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1. A drive circuit for providing a DC voltage to a plurality of light-emitting diodes (LEDs), comprising: a rectifier configured to convert an applied AC voltage to a rectified DC voltage; a passive voltage circuit configured to receive the rectified DC voltage and produce a first charging voltage; a power factor correction circuit having an input configured to receive the rectified DC voltage and having an output configured to provide a rail DC voltage; a switching DC-DC converter configured to receive the rail DC voltage and convert the rail DC voltage to an LED drive voltage and to a second charging voltage, the DC-DC converter including a controller, at least first and second semiconductor switches, and a resonant tank circuit, the semiconductor switches selectively switched by the controller to produce a switched DC voltage, the resonant tank circuit responsive to the switched DC voltage to produce the LED drive voltage, the controller having a power input terminal, the controller operable to switch the semiconductor switches only when a voltage on the power input terminal is at least as great as a controller threshold voltage; a filter capacitor coupled to provide a controller supply voltage to the power input terminal of the controller, the filter capacitor configured to receive the first charging voltage when the applied AC voltage is initially applied to the rectifier, the first charging voltage charging the capacitor to the controller threshold voltage, the capacitor receiving the second charging voltage when the controller is operable after the capacitor charges to the controller threshold voltage; and a voltage drop sensing circuit coupled to receive the first charging voltage, the voltage drop sensing circuit configured to sense when the first charging voltage decreases upon loss of the applied AC voltage, the voltage drop sensing circuit responsive to the decreasing first charging voltage to discharge the filter capacitor below the controller threshold voltage to halt the operation of the controller and thereby cease producing the LED drive voltage.
An LED driver circuit prevents LED turn-off flash by cutting power to the LED before voltage drops too low. The circuit converts AC voltage to DC, then uses a power factor correction circuit to create a DC rail voltage. A DC-DC converter, with a controller and switches, converts this rail voltage to the LED drive voltage. The controller only works if its power input is above a threshold. A filter capacitor powers the controller. A voltage drop sensing circuit monitors the initial DC voltage. When AC power is lost, the sensing circuit discharges the filter capacitor, shutting down the controller and the DC-DC converter before the LED voltage drops to the turn-off point.
2. The circuit of claim 1 , wherein the passive voltage circuit comprises a power input resistor.
The LED driver circuit uses a power input resistor within a passive voltage circuit to generate an initial charging voltage. The passive voltage circuit is configured to receive the rectified DC voltage and produce a first charging voltage. This first charging voltage is then used to charge a filter capacitor which provides power to a controller in a DC-DC converter to drive the LEDs.
3. The circuit of claim 2 , wherein the power input resistor in the passive voltage circuit includes a first terminal and a second terminal, the first terminal connected to the rectifier, the second terminal coupled to the filter capacitor and coupled to the voltage drop sensing circuit.
The LED driver circuit includes a power input resistor in its passive voltage circuit, where one end of the resistor is connected to the rectifier and the other end connects to both the filter capacitor and a voltage drop sensing circuit. This resistor helps generate an initial charging voltage that is monitored to detect power loss and prevent LED flashing by turning off the LED before it fully discharges.
4. The circuit of claim 2 , wherein: the power input resistor in the passive voltage circuit includes a first terminal and a second terminal, the first terminal connected to the rectifier, the second terminal connected to the voltage drop sensing circuit; and the passive voltage circuit further includes a Zener diode and a forward-biased diode connected in series between the second terminal of the power input resistor and the filter capacitor.
The LED driver circuit's passive voltage circuit contains a power input resistor, a Zener diode, and a forward-biased diode. One end of the resistor connects to the rectifier, while the other end connects to the voltage drop sensing circuit. The Zener diode and the forward-biased diode are in series between the resistor and the filter capacitor. This arrangement helps generate and regulate the initial voltage used to power the LED driver controller and detect power loss.
5. The circuit of claim 2 , wherein: the power input resistor in the passive voltage circuit includes a first terminal and a second terminal, the first terminal connected to the rectifier, the second terminal further connected to the voltage drop sensing circuit; and the passive voltage circuit further includes a forward-biased diode and resistor connected in series between the second terminal of the power input resistor and the filter capacitor.
The LED driver circuit utilizes a power input resistor and a forward-biased diode with a resistor in its passive voltage circuit. One end of the power input resistor connects to the rectifier, and the other end is connected to the voltage drop sensing circuit. The forward-biased diode and resistor in series connect between the resistor and the filter capacitor. This configuration assists in providing the initial charge to the filter capacitor and monitoring voltage drops for power loss detection to prevent LED flashing.
6. The circuit of claim 1 , wherein the voltage drop sensing circuit comprises a discharge resistor and a discharge transistor, the discharge resistor and the discharge transistor connected in series across the filter capacitor, the discharge transistor responsive to the decreasing first charging voltage to turn on the discharge transistor and to discharge the filter capacitor via the discharge resistor.
The LED driver circuit's voltage drop sensing circuit consists of a discharge resistor and a discharge transistor connected in series across the filter capacitor. When the circuit detects a drop in input voltage, the discharge transistor turns on, discharging the filter capacitor through the discharge resistor. This quickly cuts power to the LED driver controller, preventing the LEDs from flashing as power is lost.
7. The circuit of claim 1 , wherein the voltage drop sensing circuit further comprises a voltage sensing capacitor connected to the control terminal of the discharge transistor, the voltage sensing capacitor having a capacitance less than the capacitance of the filter capacitor, the voltage sensing capacitor discharging faster than the filter capacitor upon loss of the applied AC voltage to turn on the discharge transistor and increase the discharge rate of the filter capacitor.
The LED driver circuit's voltage drop sensing circuit includes a discharge resistor, a discharge transistor, and a voltage sensing capacitor connected to the transistor's control terminal. The voltage sensing capacitor has a smaller capacitance than the main filter capacitor. When input power is lost, the smaller capacitor discharges faster, quickly turning on the discharge transistor and rapidly discharging the larger filter capacitor. This faster discharge ensures the LED driver controller shuts down before the LEDs can flash.
8. The circuit of claim 1 , further including a capacitor coupled to the output of the power factor correction circuit, the capacitor configured to maintain the DC rail voltage on the output of the power factor correction circuit at a slowly decreasing level for a selected time after the loss of the applied AC voltage to enable the DC-DC converter to continue generating the LED drive voltage, the LED drive voltage decreasing in response to the decreasing level of the DC rail voltage, the voltage drop sensing circuit operable to halt the operation of the controller before the LED drive voltage decreases to a threshold voltage for operating the plurality of LEDs.
The LED driver circuit includes a capacitor at the output of the power factor correction (PFC) circuit to maintain the DC rail voltage for a short time after power loss, allowing the DC-DC converter to continue powering the LEDs. As the DC rail voltage slowly decreases, so does the LED drive voltage. The voltage drop sensing circuit is activated to shut down the DC-DC converter controller before the LED drive voltage falls below the LED's operational threshold, thus preventing the LEDs from flashing when power is lost.
9. A drive circuit for providing a DC voltage to a plurality of light-emitting diodes (LEDs) in response to an applied input voltage, comprising: a first charging voltage circuit responsive to the applied input voltage to generate a first charging voltage; a rail voltage circuit responsive to the applied input voltage to generate a rail voltage; a switching DC-DC converter responsive to the rail DC voltage to generate an LED drive voltage and a second charging voltage, the DC-DC converter including a controller having a power input terminal, the DC-DC converter operable only when a voltage on the power input terminal of the controller is at least as great as a controller threshold voltage; a filter capacitor coupled to provide a controller supply voltage to the power input terminal of the controller, the filter capacitor receiving the first charging voltage when the applied input voltage is active, the first charging voltage charging the filter capacitor to the controller threshold voltage, the filter capacitor receiving the second charging voltage when the controller is operable after the filter capacitor charges to the controller threshold voltage; and a voltage drop sensing circuit coupled to receive the first charging voltage, the voltage drop sensing circuit sensing when the first charging voltage decreases upon loss of the applied input voltage, the voltage drop sensing circuit responsive to the decreasing first charging voltage to discharge the filter capacitor below the controller threshold voltage to halt the operation of the controller and thereby cease producing the LED drive voltage.
An LED driver circuit prevents LED flashing during power loss by monitoring input voltage. A charging circuit generates a first charging voltage. A rail voltage circuit generates a rail voltage. A DC-DC converter uses the rail voltage to create the LED drive voltage and a second charging voltage, but requires a controller to be powered above a threshold. A filter capacitor powers the controller, charged by the first and second charging voltages. A voltage drop sensing circuit monitors the first charging voltage; when power is lost, it discharges the filter capacitor, shutting down the controller and LEDs before they can flash.
10. The circuit of claim 9 , further including a capacitor connected to the rail voltage circuit, the capacitor maintaining the DC rail voltage at a slowly decreasing level for a selected time after the loss of the applied input voltage to enable the DC-DC converter to continue generating the LED drive voltage, the LED drive voltage decreasing in response to the decreasing level of the DC rail voltage, the voltage drop sensing circuit operable to halt the operation of the DC-DC converter before the LED drive voltage decreases to a threshold voltage for operating the plurality of LEDs.
The LED driver circuit has a capacitor on the rail voltage circuit that maintains a DC rail voltage for a short time after power loss, allowing the DC-DC converter to continue powering the LEDs. As the rail voltage slowly decreases, the LED drive voltage also decreases. A voltage drop sensing circuit monitors the input voltage. When power is lost, this sensing circuit shuts down the DC-DC converter before the LED drive voltage falls below the LEDs' operational threshold. This prevents the LEDs from flashing as power is lost.
11. A method for preventing power-off flash in a light-emitting diode (LED) drive circuit, comprising: generating a switched DC voltage from an applied input voltage with a switching DC-DC converter, the switching DC-DC converter controlled by a switching controller having a power input terminal; generating an LED drive voltage from the switched DC voltage generating a first capacitor charging voltage responsive to the applied input voltage; generating a second capacitor charging voltage responsive to the switched DC voltage; applying the first capacitor charging voltage and the second capacitor charging voltage to a controller power input capacitor to charge the controller power input capacitor and provide a DC supply voltage to the switching controller; and sensing a loss of the applied input voltage and discharging the controller input capacitor to disable the switching controller before the LED drive voltage decreases to a voltage level below an operational threshold voltage of the plurality of LEDs.
A method to prevent LED power-off flash involves generating a switched DC voltage from an input voltage using a DC-DC converter controlled by a switching controller. First and second capacitor charging voltages are generated responsive to the applied input voltage and switched DC voltage respectively. The first and second voltages charge a capacitor that powers the switching controller. The method detects when the input voltage is lost and then discharges the controller's power capacitor, disabling the controller before the LED voltage drops below the level needed to keep the LEDs on and prevents them from flashing.
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August 3, 2016
September 19, 2017
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